Electrode catalyst, gas diffusion electrode-forming composition, gas diffusion electrode, membrane electrode assembly, and fuel cell stack

ABSTRACT

Provided is an electrode catalyst in which the contents of chlorine (Cl) species and bromine (Br) species are reduced to a predetermined level or lower, capable of exhibiting sufficient catalyst performance. The electrode catalyst has a core-shell structure including a support, a core part formed on the support and a shell part formed to cover at least a part of the surface of the core part. A concentration of bromine (Br) species of the electrode catalyst as measured by X-ray fluorescence (XRF) spectroscopy is 400 ppm or less, and a concentration of chlorine (Cl) species of the electrode catalyst as measured by X-ray fluorescence (XRF) spectroscopy is 900 ppm or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a U.S. national phase application under 35 U.S.C. §371 ofInternational Patent Application No. PCT/JP2016/076280, filed on Sep. 7,2016, and claims benefit of priority to Japanese Patent Application No.2015-185974, filed Sep. 18, 2015. The entire contents of theseapplications are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to an electrode catalyst. Also, this inventionrelates to a composition for forming a gas diffusion electrode, a gasdiffusion electrode, a membrane-electrode assembly, and a fuel cellstack that include the electrode catalyst.

BACKGROUND

A so-called polymer electrolyte fuel cell (Polymer Electrolyte FuelCell: hereinafter called “PEFC” as needed), has its operatingtemperature of from a room temperature to about 80° C. Also, since PEFCmakes it possible to employ inexpensive general-purpose plastics, etc.for members constituting its fuel cell body, it is possible to realizereduction in weight.

Furthermore, PEFC makes it possible to achieve thinning of a polymerelectrolyte membrane, enabling an electric resistance to be reduced,thereby enabling a power loss to be reduced relatively easily. Due toPEFC having not a few advantages as described above, it is applicable toa fuel cell vehicle, a home cogeneration system, and the like.

As an electrode catalyst for PEFC, there has been proposed an electrodecatalyst in which platinum (Pt) or a platinum (Pt) alloy, i.e., acomponent for the electrode catalyst, is supported on a carbon servingas a support (for example, Japanese Patent Application Publication No.2011-3492, MATSUOKA et al., “Degradation of Polymer Electrolyte fuelcells under the existence of anion species”, J. Power Sources, May 1,2008, Vol. 179 No. 2, P. 560-565).

Conventionally, there have been disclosed that, as for an electrodecatalyst for PEFC, if the content of chlorine contained in the electrodecatalyst is 100 ppm or more, it is not desirable as an electrodecatalyst (for example, Japanese Patent No. 4,286,499); and that this isbecause if the content of chlorine contained in the electrode catalystis 100 ppm or more, it is impossible to obtain a sufficient catalyticactivity for the electrode catalyst for fuel cells; and corrosion of itscatalyst layer will occur, thus shortening the life of the fuel cell.

Then, there is disclosed, as the catalyst component of the electrodecatalyst, a powder of platinum (Pt) or platinum (Pt) alloy that containsless than 100 ppm of chlorine (for example, Japanese Patent No.4,286,499).

As for the preparation of the powder of platinum (Pt) or platinum (Pt)alloy, there is disclosed the following method: forming a melt whichcontains, as starting materials, a chlorine-free platinum compound and achlorine-free compound of alloying elements; heating the melt up to areaction temperature at which the platinum compound and the compound ofthe alloying elements are thermally decomposed to give an oxide; coolingthe melt; and the melt is dissolved in water and the resulting oxide ormixed oxides are converted into a powder of platinum or platinum alloyby successive reduction.

Further, there is disclosed a PEFC in which part of protons of an acidgroup of an electrolyte contained in a catalyst layer of an electrode ofa membrane-electrode assembly is exchanged for a phosphonium ion,defining a compound structure such that a counter anion of thephosphonium contains no halogen elements (for example, Japanese PatentNo. 5,358,997).

The reason, as is disclosed therein. is that residues of the halogenelements in the electrode cause a degradation in cell performance.Specifically, it is described that the residues of a fluoride ion, achloride ion, or a bromide ion amongst halide ions in the electrodesometimes cause degradation in cell performance, and particularly, theresidues of the chloride ion in the electrode poison the electrodecatalyst, and cause Pt serving as a catalyst to be eluted from acatalyst layer as a complex ion such as PtCl₄ ²⁻, PtCl₆ ²⁻ to causedegradation of the cell performance.

Moreover, there is disclosed a method for producing core-shell particlesobtained by filtering a dispersion liquid having core-shell particlesdispersed in a solvent using ultrafilters or the like, cleaning andsubstituting the same with a solvent as necessary (for example, JapanesePatent No. 5,443,029). Specifically, there is disclosed in a preparationprocess of a core metal particles dispersion liquid, the core metalparticles dispersion liquid was cleaned until no Cl ions were detected.

Further, there is disclosed a method for producing carbon supportedcore-shell catalyst fine particles which controls a deposition of ashell metal material composing a shell part on a surface of a carbonsupport (for example, Japanese Patent No. 5,672,752). Furthermore, thereis disclosed a method for producing a platinum core-shell catalystcapable of directly depositing platinum on a gold core particle (forexample, Japanese Patent No. 5,660,603). In production processesprovided in these two methods for producing a electrode catalyst(core-shell catalyst), it is disclosed that the electrode catalyst(core-shell catalyst) is cleaned with extra pure water.

However, the methods for producing an electrode catalyst (core-shellcatalyst) disclosed in the aforementioned patent documents focus onchlorine only amongst halogens, and merely work on reduction/removal ofchlorine although it shows the findings that residues of halogens reducecell performance.

The applicant of the present patent application presents the followingpublications as those that describe the aforementioned inventions knownto the public through publications.

SUMMARY

As mentioned above, from the viewpoint of improving catalytic activityand lifetime as an electrode catalyst for PEFC, it is important toreduce the content of halogen, particularly of chlorine speciescontained in the catalyst.

However, the present inventors have found out that when a core-shellcatalyst is employed as an electrode catalyst for PEFC, a sufficientcatalyst performance cannot be obtained unless not only the content ofchlorine species but the content of bromine species is reduced to apredetermined level or lower, and that bromine species has a greaterimpact than chlorine species on degradation of catalyst performance.Namely, in a case where a core-shell catalyst is employed as anelectrode catalyst for PEFC, there has been room for improvement in theaforementioned conventional techniques.

This invention has been made in view of such technical circumstances,and it is an object of this invention to provide an electrode catalysthaving the contents of chlorine species and bromine species reduced topredetermined levels or lower, enabling the electrode catalyst toexhibit sufficient catalytic performance.

Further, it is another object of this invention to provide a compositionfor forming a gas diffusion electrode, a gas diffusion electrode, amembrane-electrode assembly (MEA), and a fuel cell stack that includethe aforementioned electrode catalyst.

The present inventors, as a result of having performed intensivestudies, found out that it is possible to constitute an electrodecatalyst which exhibits a satisfactory catalyst performance (acore-shell catalyst to be described later) by reducing the concentrationof chlorine (Cl) species contained in the electrode catalyst to 900 ppmor lower and by reducing the concentration of bromine (Br) speciescontained therein to 400 ppm or lower, when measured by X-rayfluorescence (XRF), and have completed this invention.

More specifically, this invention includes the following technicalmatters:

That is, this invention provides:

(1) an electrode catalyst having a core-shell structure including:

-   -   a support;    -   a core part formed on the support; and    -   a shell part formed to cover at least a part of a surface of the        core part,    -   wherein the concentration of bromine (Br) species is not higher        than 400 ppm when measured by X-ray fluorescence (XRF)        spectroscopy, and the concentration of chlorine (Cl) species is        not higher than 900 ppm when measured by X-ray fluorescence        (XRF) spectroscopy.

Since the concentrations of chlorine (Cl) species and bromine (Br)species contained in the catalyst are respectively rendered to be notgreater than 900 ppm and not greater than 400 ppm, the electrodecatalyst of this invention can exhibit a sufficient catalytic activityas an electrode catalyst.

Further, the electrode catalyst has a core-shell structure, and issuitable for reducing the manufacturing cost.

In this invention, the bromine (Br) species refers to a chemical speciescontaining bromine as a constituent element. Specifically, the chemicalspecies containing bromine include bromine atom (Br), bromine molecule(Br₂), bromide ion (Br⁻), bromine radical (Br.), polyatomic bromine ionand a bromine compound (e.g. X—Br where X represents a counterion).

In this invention, the chlorine (Cl) species refers to a chemicalspecies containing chlorine as a constituent element. Specifically, thechemical species containing chlorine include chlorine atom (Cl),chlorine molecule (Cl₂), chloride ion (Cl⁻), chlorine radical (Cl.),polyatomic chloride ion and a chlorine compound (e.g. X—Cl where Xrepresents a counterion).

In this invention, bromine (Br) species concentration and chlorine (Cl)species concentration are measured by X-ray fluorescence (XRF)spectrometry. A value of the bromine (Br) species contained in theelectrode catalyst that is measured by X-ray fluorescence (XRF)spectrometry is the concentration of bromine (Br) species. Likewise, Avalue of the chlorine (Cl) species contained in the electrode catalystthat is measured by X-ray fluorescence (XRF) spectrometry is theconcentration of chlorine (Cl) species.

Here, the bromine (Br) species concentration and chlorine (Cl) speciesconcentration are concentrations of the bromine atoms and chlorine atomsin terms of the bromine element and chlorine element that arerespectively contained in the electrode catalyst.

As stated above, with respect to the core-shell catalyst, the presentinventors also focused on bromine (Br) species other than chlorine (Cl)species, and found out that it is important to sufficiently remove themas impurities.

Bromine being a halogen element as with chlorine, is an element from thesame family (7B) as chlorine, and they have analogous physicalproperties as represented by e.g., their ion radii. For this reason, abromine-containing metal compound is sometimes used as a raw materialfor the core part or shell part of a core-shell catalyst, instead of achloride salt of palladium (Pd) and platinum (Pt). Further, thebromine-containing metal compound is sometimes used as a precursor forproducing the chloride salt of platinum (Pt) that is employed as a rawmaterial of the core-shell catalyst. Moreover, during a productionprocess, the bromine (Br) species is sometimes unintentionally mixedwith the core-shell catalyst, adhering to the electrode catalyst as animpurity.

Further, the present inventors found out that the bromine (Br) specieshas a greater impact on the degradation of catalyst performance, thanthe chloride (Cl) species. From this viewpoint, in the electrodecatalyst of this invention as set forth in (1),

(2) the bromine (Br) species concentration is preferably not greaterthan 300 ppm, and

(3) the bromine (Br) species concentration is more preferably notgreater than 200 ppm.

Thus can be achieved the effects of this invention more reliably.

Further, from the viewpoint of more reliably achieving the effects ofthis invention, it is preferable to reduce as much chlorine (Cl) speciesas possible, specifically,

(4) the chlorine (Cl) species concentration is preferably less than 900ppm, more preferably not greater than 800 ppm, and still more preferablynot greater than 600 ppm.

Further, in this invention,

(5) the chlorine (Cl) species concentration may be equal to or higherthan 0 ppm.

Here, in this invention, “the chlorine (Cl) species concentration is 0ppm” denotes a state in which the chloride (Cl) species measured throughX-ray fluorescence (XRF) spectroscopy, is reduced to a level at whichthe chloride (Cl) species is not detected (undetected). Although it isideal that the chlorine (Cl) species is thoroughly removed in order toimprove the catalyst performance, the present inventors confirmed thatthe catalyst performance sufficiently improves by reducing the chloride(Cl) species to a level at which the chloride (Cl) species measuredthrough X-ray fluorescence (XRF) spectroscopy, is not detected(undetected). In the case of this invention, the detection limit of thechlorine (Cl) species measured through X-ray fluorescence (XRF)spectroscopy, is 100 ppm. Consequently, in the case that “the chlorine(Cl) species concentration is 0 ppm”, there is a possibility that thechlorine (Cl) species may be contained in a concentration range of lessthan 100 ppm.

Further, the bromine (Br) species concentration may be equal to orhigher than 0 ppm. Similarly, with regard to the bromine (Br) speciesconcentration, “the bromine (Br) species concentration is 0 ppm” denotesa state in which the bromine (Br) species measured through X-rayfluorescence (XRF) spectroscopy, is reduced to a level at which thebromine (Br) species is not detected (undetected). As to this bromine(Br) species concentration as well, the present inventors confirmed thatthe catalyst performance has sufficiently improved by reducing thebromine (br) species to a level at which the bromine (Br) speciesmeasured through X-ray fluorescence (XRF) spectroscopy, is not detected(undetected). In the case of this invention, the detection limit of thebromine (Br) species measured through X-ray fluorescence (XRF)spectroscopy, is 100 ppm. Consequently, in the case that “the bromine(Br) species concentration is 0 ppm”, there is a possibility that thebromine (Br) species may be contained in a concentration range of lessthan 100 ppm.

Further, in the electrode catalysts (1) to (5) of this invention, if thebromine (Br) species and the chlorine (Cl) species concentrations havebeen respectively reduced to not greater than 400 ppm and not greaterthan 900 ppm,

(6) the chlorine (Cl) species concentration may be not less than 100ppm. Namely, the chlorine (Cl) species concentration may be from 100 ppmto 900 ppm.

In this case as well, the effects of this invention can be attained.According to this structure, the chlorine (Cl) species concentration isnot reduced to less than 100 ppm, thus making it possible to reducecosts and labors for reducing the chlorine in the production process.

Further, this invention provides

(7) the electrode catalyst set forth in any one of (1) to (6), whereinthe core-shell structure includes: the core part; and a single-layeredshell part having the shell part formed to cover at least a part of thesurface of the core part.

In this case as well, the effects of this invention can be attained. Byemploying the aforementioned structure, the electrode catalyst of thisinvention may reduce the content of a noble metal (s) such as platinumused in the core part, thereby enabling reduction in raw material cost.

According to the structure of the invention (7), namely,

(8) in a case of the shell part being of the single-layered structure,it is preferable that the shell part contain at least one metal selectedfrom platinum (Pt) and a platinum (Pt) alloy. This makes it possible tomore easily obtain a superior catalytic activity.

Further, according to the structure of the invention (8), namely,

(9) in a case of the shell part being of the single-layered structure,it is preferable that the core part contain at least one kind of metalselected from the group consisting of palladium (Pd), a palladium (Pd)alloy, a platinum (Pt) alloy, gold (Au), nickel (Ni) and a nickel (Ni)alloy. This makes it possible to more reliably obtain the effects ofthis invention. Furthermore, by employing the aforementioned structure,a higher catalyst activity and a higher durability can be obtained.

Further, when this invention has the structure (8), namely,

(10) in a case of the shell part being of the single-layered structure,the core part may contain one or more metal elements other than noblemetals as a main component(s). In this structure as well, the effects ofthis invention can be attained. Furthermore, by employing thisstructure, cost reduction can be easily achieved due to reduction innoble metal content.

Further, this invention provides the electrode catalyst as set forth inany one of (7) to (9), in which,

(11) in a case of the shell part being of the single-layered structure,the support contains an electrically conductive carbon, the shell partcontains platinum (Pt), and the core part contains palladium (Pd).

Thus, the effects of this invention can be achieved more reliably.Further, by employing the abovementioned structure, there can beachieved a higher catalytic activity and a higher durability.Furthermore, by employing the abovementioned structure, the electrodecatalyst of this invention, as compared to conventional electrodecatalysts having a structure where platinum is supported on a carbonsupport, is capable of reducing the amount of platinum contained, and isthus capable of easily reducing a raw material cost.

Furthermore, according to this invention,

(12) there is provided the electrode catalyst as set forth in any one of(1) to (6),

-   -   in which the core-shell structure has:    -   the core part; and    -   a two-layered shell part having a first and a second shell parts        formed such that the first shell part covers at least a part of        the surface of the core part, and the second shell part covers        at least a part of a surface of the first shell part.

Thus, the effects of this invention can be achieved more reliably. Byemploying the abovementioned structure, the electrode catalyst of thisinvention may reduce the contained amount of a noble metal(s) such asplatinum used in the core part, and is thus capable of easily reducing araw material cost.

When this invention employs the structure of the invention (12), namely,

(13) in a case of the shell part being of the two-layered structure, itis preferable that the second shell part contain at least one metalselected from platinum (Pt) and a platinum (Pt) alloy. This makes itpossible to more easily obtain a superior catalytic activity.

Further, when this invention employs the structure of the invention(13), namely,

(14) in a case of the shell part being of the two-layered structure, itis preferable that the first shell part contain at least one kind ofmetal selected from the group consisting of palladium (Pd), a palladium(Pd) alloy, a platinum (Pt) alloy, gold (Au), nickel (Ni) and a nickel(Ni) alloy. In this way, the effects of this invention can be achievedmore reliably. Furthermore, employing the aforementioned structure makesit possible to obtain a higher catalytic activity and a higherdurability.

Further, when this invention has the structure (14), namely,

(15) in a case of the shell part being of the two-layered structure, itis preferable that the core part contain one or more metal elementsother than noble metals as a main component(s). In this structure aswell, the effects of this invention can be attained.

Furthermore, by employing this structure, cost reduction can be moreeasily achieved due to reduction in noble metal content.

Also, this invention provides the electrode catalyst as set forth in(13) to (15) in which

(16) in a case of the shell part being of the two-layered structure, thefirst shell part contains palladium (Pd), and the second shell partcontains platinum (Pt).

In this way, the effects of this invention can be achieved morereliably. By employing the abovementioned structure, there can beachieved a higher catalytic activity and a higher durability.

Further, this invention provides

(17) a composition for forming a gas diffusion electrode, including theelectrode catalyst as set forth in any one of (1) to (16).

According to the gas diffusion electrode-forming composition of thisinvention, it is possible to easily produce a gas diffusion electrodewith a high catalytic activity (polarization property) because itcontains the electrode catalyst of this invention.

Furthermore, this invention provides

(18) a gas diffusion electrode containing the electrode catalyst as setforth in any one of (1) to (16).

According to the gas diffusion electrode of this invention, it ispossible to achieve a high catalytic activity (polarization property)because it contains the electrode catalyst of this invention.

Furthermore, this invention provides

(19) a membrane-electrode assembly (MEA) including the gas diffusionelectrode as set forth in (18).

According to the membrane-electrode assembly (MEA) of this invention, itis possible to achieve a high battery property because it contains thegas diffusion electrode of this invention.

Still further, this invention provides

(20) a fuel cell stack including the membrane-electrode assembly (MEA)as set forth in (19).

According to the fuel cell stack of this invention, it is possible toachieve a high battery property because it contains themembrane-electrode assembly (MEA) of this invention.

According to this invention, there can be provided an electrode catalystthat can exhibit a sufficient catalytic activity, because theconcentrations of chlorine (Cl) species and bromine (Br) speciescontained in the electrode catalyst are respectively rendered to be notgreater than 900 ppm (preferably less than 900 ppm) and not greater than400 ppm (preferably not greater than 300 ppm, more preferably notgreater than 200 ppm).

Further, according to this invention, there can be provided an electrodecatalyst that is also suitable for reduction of the manufacturing costbecause the electrode catalyst has the core-shell structure.

Further, according to this invention, there can be provided acomposition for forming a gas diffusion electrode, a gas diffusionelectrode, a membrane-electrode assembly (MEA), and a fuel cell stackthat include the aforementioned electrode catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing an example of the electrodecatalyst (core-shell catalyst) of this invention.

FIG. 2 is a schematic sectional view showing another example of theelectrode catalyst (core-shell catalyst) of this invention.

FIG. 3 is a schematic sectional view showing another example of theelectrode catalyst (core-shell catalyst) of this invention.

FIG. 4 is a schematic sectional view showing another example of theelectrode catalyst (core-shell catalyst) of this invention.

FIG. 5 is a schematic diagram showing an example of a fuel cell stack ofthis invention.

FIG. 6 is a schematic diagram showing a schematic configuration of arotating disk electrode measuring device equipped with a rotating discelectrode used in a working example.

DETAILED DESCRIPTION

Examples of this invention are described in detail hereunder withreference to the drawings when necessary.

FIG. 1 is a schematic cross-sectional view showing a preferableembodiment of an electrode catalyst (core-shell catalyst) of thisinvention.

As shown in FIG. 1, an electrode catalyst 1 of this invention includes asupport 2; and catalyst particles 3 supported on the support 2 andhaving a so-called “core-shell structure.” Each catalyst particle 3 hasa core part 4; and a shell part 5 covering at least a part of thesurface of the core part 4. The catalyst particles 3 thus have aso-called “core-shell structure” including the core part 4 and the shellpart 5 formed on the core part 4.

That is, the electrode catalyst 1 has the catalyst particles 3 supportedon the support 2, and the catalyst particles 3 have the structure wherethe core part 4 serves as a core (core portion), and the shell part 5 asa shell covers at least a part of the surface of the core part 4.

Further, the constituent element (chemical composition) of the core part4 and the constituent element (chemical composition) of the shell part 5differ from each other in composition.

There are no particular restrictions on the electrode catalyst 1 of thisinvention except that the shell part 5 has to be formed on at least apart of the surface of the core part 4 of each catalyst particle 3.

For example, in terms of more reliably achieving the effects of thisinvention, it is preferred that the electrode catalyst 1 be in a statewhere the whole range of the surface of the core part 4 is substantiallycovered by the shell part 5, as shown in FIG. 1.

Further, the electrode catalyst 1 may also be in a state where a part ofthe surface of the core part 4 is covered by the shell part 5, and therest part of the surface of the core part 4 is thus partially exposed,provided that the effects of this invention can be achieved.

That is, with regard to the electrode catalyst of this invention, it issufficient that the shell part be formed on at least a part of thesurface of the core part.

FIG. 2 is a schematic cross-sectional view showing another preferableembodiment (electrode catalyst 1A) of the electrode catalyst (core-shellcatalyst) of this invention.

As shown in FIG. 2, an electrode catalyst 1A of this invention hascatalyst particles 3 a each being composed of a core part 4; a shellpart 5 a covering a part of the surface of the core part 4; and a shellpart 5 b covering an other part of the surface of the core part 4.

With regard to the catalyst particles 3 a contained in the electrodecatalyst 1A shown in FIG. 2, there is a part of the core part 4 that isneither covered by the shell part 5 a nor covered by the shell part 5 b.This part of the core part 4 composes a core part-exposed surface 4 s.

That is, as shown in FIG. 2, so far as the effects of this invention canbe achieved, the catalyst particles 3 a contained in the electrodecatalyst 1A may also be in a state where the surface of the core part 4is partially exposed (e.g. a state where 4 s as a part of the surface ofthe core part 4 shown in FIG. 2 is exposed).

In other words, as is the case with the electrode catalyst 1A shown inFIG. 2, the shell part 5 a may be partially formed on a part of thesurface of the core part 4, and the shell part 5 b may then be partiallyformed on another part of the surface of the core part 4.

FIG. 3 is a schematic cross-sectional view showing another preferableembodiment (electrode catalyst 1B) of the electrode catalyst (core-shellcatalyst) of this invention.

As shown in FIG. 3, an electrode catalyst 1B of this invention hascatalyst particles 3 each being composed of a core part 4; and a shellpart 5 substantially covering the whole range of the surface of the corepart 4.

The shell part 5 may have a two-layered structure composed of a firstshell part 6 and a second shell part 7. That is, the catalyst particles3 have a so-called “core-shell structure” comprised of the core part 4;and the shell part 5 (first shell part 6 and second shell part 7) formedon the core part 4.

The electrode catalyst 1B has a structure where the catalyst particles 3are supported on the support 2, having the core part 4 serving as a core(core portion); and the whole range of the surface of the core part 4 issubstantially covered by the shell part 5 composed of the first shellpart 6 and the second shell part 7.

Here, the constituent element (chemical composition) of the core part 4,the constituent element (chemical composition) of the first shell part 6and the constituent element (chemical composition) of the second shellpart 7 differ from one another in composition.

Moreover, the shell part 5 included in the electrode catalyst 1B of thisinvention may further include another shell part in addition to thefirst shell part 6 and the second shell part 7.

In terms of more reliably achieving the effects of this invention, it ispreferred that the electrode catalyst 1B be in a state where the wholerange of the surface of the core part 4 is substantially covered by theshell part 5, as shown in FIG. 3.

FIG. 4 is a schematic cross-sectional view showing another preferableembodiment (electrode catalyst 1C) of the electrode catalyst (core-shellcatalyst) of this invention.

As shown in FIG. 4, an electrode catalyst 1C of this invention hascatalyst particles 3 a each being composed of a core part 4; a shellpart 5 a covering a part of the surface of the core part 4; and a shellpart 5 b covering another part of the surface of the core part 4.

The shell part 5 a may have a two-layered structure composed of a firstshell part 6 a and a second shell part 7 a.

Further, the shell part 5 b may have a two-layered structure composed ofa first shell part 6 b and a second shell part 7 b.

That is, the catalyst particles 3 a have a so-called “core-shellstructure” including the core part 4; the shell part 5 a (first shellpart 6 a and second shell part 7 a) formed on the core part 4; and theshell part 5 b (first shell part 6 b and second shell part 7 b) formedon the core part 4.

With regard to the shell part 5 b composing the catalyst particle 3 ashown in FIG. 4, there is a part of the first shell part 6 b that is notcovered by the second shell part 7 b. The part of the first shell part 6b that is not covered by the second shell part 7 b composes a firstshell part-exposed surface 6 s.

With regard to the shell part 5 a composing the catalyst particle 3 ashown in FIG. 4, it is preferred that the whole range of the first shellpart 6 a be substantially covered by the second shell part 7 a.

Further, as shown in FIG. 4 and with regard to the shell part 5 bcomposing each catalyst particle 3 a, also permissible is a state wherea part of the surface of the first shell part 6 b is covered, and thesurface of the first shell part 6 b is thus partially exposed (e.g. astate shown in FIG. 4 where the part 6 s of the surface of the firstshell part 6 b is exposed), so far as the effects of this invention canbe achieved.

Moreover, on the premise that the effects of this invention can beachieved, the electrode catalyst 1 may allow a “complex of the core part4 and shell part 5 with the whole range of the surface of the core part4 being substantially covered by the shell part 5” and a “complex of thecore part 4 and shell part 5 with the surface of the core part 4 beingpartially covered by the shell part 5” to coexist on the support 2 in amixed manner.

Specifically, the electrode catalyst of this invention may be in a statewhere the electrode catalysts 1 and 1A shown in FIGS. 1 and 2 and theelectrode catalysts 1B and 1C shown in FIGS. 3 and 4 coexist in a mixedmanner, provided the effects of this invention can be achieved.

Further, the electrode catalyst of this invention may allow the shellpart 5 a and the shell part 5 b to coexist in a mixed manner withrespect to an identical core part 4, as shown in FIG. 4, so far as theeffects of this invention can be achieved.

Furthermore, so far as the effects of this invention can be achieved,the electrode catalyst of this invention may allow only the shell part 5a to exist with respect to an identical core part 4 or only the shellpart 5 b to exist with respect to an identical core part 4 (none ofthese states are shown in the drawings).

Furthermore, so far as the effects of this invention can be achieved,the electrode catalyst 1 may also be in a state where “particles onlycomposed of the core parts 4 that are not covered by the shell parts 5”are supported on the support 2, in addition to at least one kind of theelectrode catalysts 1, 1A, 1B and 1C (not shown).

Furthermore, on the premise that the effects of this invention can beachieved, the electrode catalyst 1 may also be in a state where“particles only composed of the constituent element of the shell part 5”are supported on the support 2 without being in contact with the coreparts 4, in addition to at least one kind of the electrode catalysts 1,1A, 1B and 1C (not shown).

Furthermore, on the premise that the effects of this invention can beachieved, the electrode catalyst 1 may also be in a state where“particles only composed of the core parts 4 that are not covered by theshell parts 5” and “particles only composed of the constituent elementof the shell part 5” are individually and independently supported on thesupport 2, in addition to at least one kind of the electrode catalysts1, 1A, 1B and 1C.

It is preferred that the core part 4 have an average particle diameterof 2 to 40 nm, more preferably 4 to 20 nm, particularly preferably 5 to15 nm.

As for the thickness of the shell part 5 (thickness from the surface incontact with the core part 4 to the outer surface of the shell part 5),a preferable range thereof is to be appropriately determined based onthe design concept(s) of the electrode catalyst.

For example, when the amount of the metal element (e.g. platinum) usedto compose the shell part 5 is intended to be minimized, a layercomposed of one atom (one atomic layer) is preferred. In this case, whenthere is only one kind of metal element composing the shell part 5, itis preferred that the thickness of the shell part 5 be twice as large asthe diameter of one atom of such metal element (in sphericalapproximation). Further, when there are not fewer than two kinds ofmetal elements composing the shell part 5, it is preferred that thethickness of the shell part 5 be that of a layer of one atom (one atomiclayer formed with two or more kinds of atoms being apposed on thesurface of the core part 4).

Further, for example, when attempting to improve a durability byemploying a shell part 5 of a larger thickness, it is preferred thatsuch thickness be 1 to 10 nm, more preferably 2 to 5 nm.

When the shell part 5 has the two-layered structure composed of thefirst shell part 6 and the second shell part 7, preferable ranges of thethicknesses of the first shell part 6 and second shell part 7 areappropriately determined based on the design concept(s) of the electrodecatalyst of this invention.

For example, when the amount of a noble metal such as platinum (Pt) as ametal element contained in the second shell part 7 is intended to beminimized, it is preferred that the second shell part 7 be a layercomposed of one atom (one atomic layer). In this case, when there isonly one kind of metal element composing the second shell part 7, it ispreferred that the thickness of the second shell part 7 be approximatelytwice as large as the diameter of one atom of such metal element(provided that an atom is considered as a sphere).

Further, when there are not fewer than two kinds of metal elementscontained in the second shell part 7, it is preferred that the secondshell part 7 have a thickness equivalent to that of a layer composed ofnot fewer than one kind of atom (one atomic layer formed with two ormore kinds of atoms being apposed in the surface direction of the corepart 4). For example, when attempting to improve the durability of theelectrode catalyst by employing a second shell part 7 of a largerthickness, it is preferred that the thickness of the second shell part 7be 1.0 to 5.0 nm. If the durability of the electrode catalyst is to befurther improved, it is preferred that the thickness of the second shellpart 7 be 2.0 to 10.0 nm.

Here, in this invention, “average particle diameter” refers to anaverage value of the diameters of an arbitrary number of particles asparticle groups that are observed through electron micrographs.

There are no particular restrictions on the support 2, as long as suchsupport 2 is capable of supporting the catalyst particles 3 as thecomplexes composed of the core parts 4 and the shell parts 5, and has alarge surface area.

Moreover, it is preferred that the support 2 be that exhibiting afavorable dispersibility and a superior electrical conductivity in acomposition used to form a gas diffusion electrode having the electrodecatalyst 1.

The support 2 may be appropriately selected from carbon-based materialssuch as glassy carbon (GC), fine carbon, carbon black, black lead,carbon fiber, activated carbon, ground product of activated carbon,carbon nanofiber and carbon nanotube; and glass-based or ceramic-basedmaterials such as oxides.

Among these materials, carbon-based materials are preferred in terms oftheir adsorptivities with respect to the core part 4 and in terms of aBET specific surface area of the support 2.

Further, as a carbon-based material, an electrically conductive carbonis preferred. Particularly, an electrically conductive carbon black ispreferred as an electrically conductive carbon. Examples of suchelectrically conductive carbon black include products by the names of“Ketjenblack EC300 J,” “Ketjenblack EC600” and “Carbon EPC” (produced byLion Corporation).

There are no particular restrictions on the component of the core part4, as long as the component is capable of being covered by the shellpart 5.

When the shell part 5 employs a single-layered structure as are thecases with the electrode catalysts 1 and 1A that are shown in FIGS. 1and 2 instead of the two-layered structure, from the viewpoint ofrelatively easily obtaining a superior catalytic activity, it ispreferable that the core part 4 include a noble metal(s) as a maincomponent(s). The core part 4 composing the catalyst particles 3 and 3 aof the electrode catalysts 1 and 1A, contains at least one kind of metalselected from the group consisting of palladium (Pd), a palladium (Pd)alloy, a platinum (Pt) alloy, gold (Au), nickel (Ni) and a nickel (Ni)alloy.

There are no particular restrictions on a palladium (Pd) alloy, as longas the alloy is to be obtained by combining palladium (Pd) with anothermetal capable of forming an alloy when combined with palladium (Pd). Forexample, such palladium (Pd) alloy may be a two-component palladium (Pd)alloy obtained by combining palladium (Pd) with another metal; or athree or more-component palladium (Pd) alloy obtained by combiningpalladium (Pd) with not fewer than two kinds of other metals.Specifically, examples of such two-component palladium (Pd) alloyinclude gold palladium (PdAu), silver palladium (PdAg) and copperpalladium (PdCu). One example of a three-component palladium (Pd) alloyis gold-silver-palladium (PdAuAg).

There are no particular restrictions on a platinum (Pt) alloy, as longas the alloy is to be obtained by combining platinum (Pt) with anothermetal capable of forming an alloy when combined with platinum (Pt). Forexample, such platinum (Pt) alloy may be a two-component platinum (Pt)alloy obtained by combining platinum (Pt) with another metal; or a threeor more-component platinum (Pt) alloy obtained by combining platinum(Pt) with not fewer than two kinds of other metals. Specifically,examples of such two-component platinum (Pt) alloy include nickelplatinum (PtNi) and cobalt platinum (PtCo).

There are no particular restrictions on a nickel (Ni) alloy, as long asthe alloy is to be obtained by combining nickel (Ni) with another metalcapable of forming an alloy when combined with nickel (Ni). For example,such nickel (Ni) alloy may be a two-component nickel (Ni) alloy obtainedby combining nickel (Ni) with another metal; or a three ormore-component nickel (Ni) alloy obtained by combining nickel (Ni) withnot fewer than two kinds of other metals. Specifically, one example ofsuch two-component nickel (Ni) alloy is tungsten nickel (NiW).

The shell part 5 contains at least one kind of metal selected fromplatinum (Pt) and a platinum (Pt) alloy. There are no particularrestrictions on a platinum (Pt) alloy, as long as the alloy is to beobtained by combining platinum (Pt) with an other metal capable offorming an alloy when combined with platinum (Pt). For example, suchplatinum (Pt) alloy may be a two-component platinum (Pt) alloy obtainedby combining platinum (Pt) with an other metal; or a three ormore-component platinum (Pt) alloy obtained by combining platinum (Pt)with not fewer than two kinds of other metals. Specifically, examples ofsuch two-component platinum (Pt) alloy include nickel platinum (PtNi),cobalt platinum (PtCo), platinum ruthenium (PtRu), platinum molybdenum(PtMo) and platinum titanium (PtTi). Particularly, in order for theshell part 5 to have a poisoning resistance against carbon monoxide, itis preferred that a platinum ruthenium (PtRu) alloy be used.

Further, when the shell part 5 employs the single-layered structure asare the cases with the electrode catalysts 1 and 1A that are shown inFIGS. 1 and 2 instead of the two-layered structure, from the perspectiveof reducing the cost for producing the electrode catalyst 1, it ispreferred that the core part 4 include a metal element(s) other thannoble metals as a main component(s) (the amount of the main component(s)is preferably not less than 60% by weight, more preferably not less than70% by weight, further preferably not less than 80% by weight of thecore part 4).

Specifically, when the shell part 5 employs the single-layeredstructure, it is preferred that the core part 4 contain, as a maincomponent(s) thereof, a metal(s) including a metal element(s) other thannoble metals, a metal nitride of such metal, a metal carbide of suchmetal, a metal oxide of such metal, an alloy containing such metal (asolid solution containing such metal and an intermetallic compoundcontaining such metal), and/or a mixture of such metal(s) and such metalcompound (s) (the amount of the main component(s) is preferably not lessthan 60% by weight, more preferably not less than 70% by weight, andfurther preferably not less than 80% by weight of the core part 4). Inthis case, it is preferred that the metal elements other than noblemetals be metal elements other than Pt, Pd, Au, Ag, Rh, Ir, Ru and Os.

Further, in this case, it is preferred that the metal nitride be atleast one kind selected from the group of Ti nitride, Zr nitride, Tanitride, Nb nitride and W nitride.

Moreover, in this case, it is preferred that the metal carbide be atleast one kind selected from the group of Ti carbide, Zr carbide, Tacarbide, Nb carbide and W carbide.

Furthermore, in this case, it is preferred that the metal oxide be atleast one kind selected from the group of Ti oxide, Zr oxide, Ta oxide,Nb oxide and W oxide.

Further, as the electrode catalysts 1B and 1C illustrated in FIGS. 3 and4, when the shell part 5 employs the two-layered structure composed ofthe first shell part 6 and the second shell part 7, it is preferred,especially from the perspective of reducing the cost for producing theelectrode catalyst 1, that the core part 4 contain a metal element(s)other than noble metals as a main component(s) (the amount of maincomponent is preferably not less than 60% by weight, more preferably notless than 70% by weight, further preferably not less than 80% by weightof the core part 4).

Specifically, when the shell part 5 employs the two-layered structure,it is preferred that the core part 4 contain, as a main component(s)thereof, a metal(s) including a metal element(s) other than noblemetals, a metal nitride of such metal, a metal carbide of such metal, ametal oxide of such metal, an alloy containing such metal (a solidsolution containing such metal and an intermetallic compound containingsuch metal), and/or a mixture of such metal(s) and such metal compound(the amount of the main component(s) is preferably not less than 60% byweight, more preferably not less than 70% by weight, and furtherpreferably not less than 80% by weight of the core part 4). In thiscase, it is preferred that the metal elements other than noble metals bemetal elements other than Pt, Pd, Au, Ag, Rh, Ir, Ru and Os.

Further, in this case, it is preferred that the metal nitride be atleast one kind selected from the group of Ti nitride, Zr nitride, Tanitride, Nb nitride and W nitride.

Moreover, in this case, it is preferred that the metal carbide be atleast one kind selected from the group of Ti carbide, Zr carbide, Tacarbide, Nb carbide and W carbide.

Furthermore, in this case, it is preferred that the metal oxide be atleast one kind selected from the group of Ti oxide, Zr oxide, Ta oxide,Nb oxide and W oxide.

A supported amount of the platinum (Pt) contained in the shell part 5 is5 to 30% by weight, preferably 8 to 25% by weight with respect to theweight of the electrode catalyst 1. It is preferred that the amount ofthe platinum (Pt) supported be not smaller than 5% by weight, becausethe electrode catalyst can fully exert its catalytic activity in suchcase. It is also preferred that the amount of the platinum (Pt)supported be not larger than 30% by weight, because the amount ofplatinum (Pt) used is thus reduced in such case, which is favorable interms of production cost.

In the case where the shell part 5 has the two-layered structurecomposed of the first shell part 6 and the second shell part 7, it ispreferred that the first shell part 6 contain at least one kind of metalselected from the group consisting of palladium (Pd), a palladium (Pd)alloy, a platinum (Pt) alloy, gold (Au), nickel (Ni) and a nickel (Ni)alloy, and it is more preferred that the first shell part 6 containelemental palladium (Pd).

From the perspective of further improving the catalytic activities ofthe electrode catalysts 1B and 1C and more easily obtaining the same, itis preferred that the first shell part 6 be mainly composed of palladium(Pd) simple substance (not less than 50 wt %), and it is more preferredthat such first shell part 6 be only composed of palladium (Pd) simplesubstance.

It is preferred that the second shell part 7 contain at least one kindof metal selected from platinum (Pt) and a platinum (Pt) alloy, and itis more preferred that such shell part 7 contain platinum (Pt) simplesubstance.

From the perspective of further improving the catalytic activities ofthe electrode catalysts 1B and 1C and more easily obtaining the same, itis preferred that the second shell part 7 be mainly composed of platinum(Pt) simple substance (not less than 50 wt %), and it is more preferredthat such second shell part 7 be only composed of platinum (Pt) simplesubstance.

Concentration of bromine (Br) species and concentration of chlorine (Cl)species The electrode catalyst 1 exhibits a bromine (Br) speciesconcentration of not greater than 400 ppm (0 to 400 ppm), preferably notgreater than 300 ppm (0 to 300 ppm), more preferably not greater than200 ppm (0 to 200 ppm) when measured through X-ray fluorescence (XRF)spectroscopy. Further, the electrode catalyst 1 satisfies theabovementioned conditions of the bromine (Br) species concentration, anda chlorine (Cl) species concentration of not greater than 900 ppm (0 to900 ppm) when measured through the same analytical method. The chlorine(Cl) species concentration is preferably less than 900 ppm (not lessthan 0 ppm and not greater than 900 ppm), more preferably not greaterthan 800 ppm (0 to 800 ppm), further preferably not greater than 600 ppm(0 to 600 ppm).

The electrode catalyst 1 is capable of fully exerting its catalyticactivity as an electrode catalyst by concurrently fulfilling theabovementioned conditions of the chloride (Cl) species concentration andthe bromine (Br) species concentration.

Here, the bromine (Br) species concentration and the chlorine (Cl)species concentration are measured through X-ray fluorescence (XRF)spectroscopy. A value obtained by measuring the bromine (Br) speciescontained in the electrode catalyst through X-ray fluorescence (XRF)spectroscopy is the bromine (Br) species concentration. Similarly, avalue obtained by measuring the chlorine (Cl) species contained in theelectrode catalyst through X-ray fluorescence (XRF) spectroscopy is thechlorine (Cl) species concentration.

Here, the bromine (Br) species concentration and the chlorine (Cl)species concentration are respectively the concentrations of the bromineatoms and chlorine atoms in terms of the bromine and chlorine elementscontained in the electrode catalyst.

X-ray fluorescence (XRF) spectroscopy is a method where a specimencontaining a particular element A is irradiated with a primary X-ray togenerate a fluorescent X-ray of such element A, followed by measuringthe intensity of such fluorescent X-ray of the element A such thatquantitative analysis of the captioned element A contained in thespecimen can be performed. When performing quantitative analysis throughX-ray fluorescence (XRF) spectroscopy, there may be employed thefundamental parameter method (FP method) used in theoretical operation.

The FP method applies the idea that if the compositions and kinds of theelements contained in a specimen are all known, the fluorescent X-ray(XRF) intensities thereof can be individually and theoreticallycalculated. In addition, the FP method allows there to be estimated acomposition(s) corresponding to the fluorescent X-ray (XRF) of eachelement that is obtained by measuring the specimen.

X-ray fluorescence (XRF) spectroscopy is performed using generalfluorescent X-ray (XRF) analyzers such as an energy dispersivefluorescent X-ray (XRF) analyzer, a scanning-type fluorescent X-ray(XRF) analyzer and a multi-element simultaneous-type fluorescent X-ray(XRF) analyzer. A fluorescent X-ray (XRF) analyzer is equipped with asoftware which makes it possible to perform experimental data processingregarding the correlation between the intensity of the fluorescent X-ray(XRF) of the element A and the concentration of the element A.

There are no particular restrictions on such software, as long as thesoftware is that generally used to perform X-ray fluorescence (XRF)spectroscopy.

For example, there may be employed a software for use in a generalfluorescent X-ray (XRF) analyzer adopting the FP method, such as ananalysis software: “UniQuant 5.” Here, one example of the abovementionedfluorescent X-ray (XRF) analyzer is a full-automatic wavelengthdispersive fluorescent X-ray analyzer (product name: Axios by SpectrisCo., Ltd.).

In order to achieve a bromine (Br) species concentration of not greaterthan 400 ppm when measured by the X-ray fluorescence (XRF) spectroscopy,it is required that a metal compound as a starting material of theelectrode catalyst 1 and a reagent(s) used in each production step ofthe electrode catalyst 1 be carefully selected. Specifically, there may,for example, be used a metal compound that does not generate bromine(Br) species, as the metal compound serving as the starting material ofthe electrode catalyst 1. Further, there may, for example, be employed acompound(s) that do not contain bromine (Br) species, as the reagent(s)used in the production steps of the electrode catalyst 1.

In order to achieve a chlorine (Cl) species concentration of not greaterthan 900 ppm when measured by the abovementioned X-ray fluorescence(XRF) spectroscopy, it is required that a metal compound as a startingmaterial of the electrode catalyst 1 and reagents used in productionsteps of the electrode catalyst be carefully selected. Specifically,there may, for example, be used a metal compound that does not generatechlorine (Cl) species, as the metal compound serving as the startingmaterial of the electrode catalyst 1. Further, there may, for example,be employed compounds that do not contain chlorine (Cl) species, as thereagents used in the production steps of the electrode catalyst 1.

Further, chlorine (Cl) species can be reduced to equal to or less than900 ppm by employing the chlorine reduction methods described later.

A production method of the electrode catalyst 1 includes a step ofproducing an electrode catalyst precursor; and a step of washing suchcatalyst precursor to meet the condition where the bromine (Br) speciesconcentration measured by the X-ray fluorescence (XRF) spectroscopy isnot greater than 400 ppm, and the chlorine (Cl) species concentrationmeasured by the same method is 0 to 900 ppm.

The electrode catalyst precursor of the electrode catalyst 1 is producedby having the support 2 support the catalyst components (core part 4,shell part 5) of the electrode catalyst.

There are no particular restrictions on a production method of theelectrode catalyst precursor as long as the method allows the catalystcomponents of the electrode catalyst 1 to be supported on the support 2.

Examples of the production method of the electrode catalyst precursorinclude an impregnation method where a solution containing the catalystcomponents of the electrode catalyst 1 is brought into contact with thesupport 2 to impregnate the support 2 with the catalyst components; aliquid phase reduction method where a reductant is put into a solutioncontaining the catalyst components of the electrode catalyst 1; anelectrochemical deposition method such as under-potential deposition(UPD); a chemical reduction method; a reductive deposition method usingadsorption hydrogen; a surface leaching method of alloy catalyst;immersion plating; a displacement plating method; a sputtering method;and a vacuum evaporation method.

Concentration of bromine (Br) species and concentration of chlorine (Cl)speNext, the concentrations of the bromine (Br) species and chlorine(Cl) species of the electrode catalyst precursor are adjusted to meetthe condition where the bromine (Br) species concentration measured bythe X-ray fluorescence (XRF) spectroscopy is not greater than 400 ppm,and the chlorine (Cl) species concentration measured by the same methodis 0 to 900 ppm. Specifically, there are employed the following chlorinereduction methods 1 to 3.

Chlorine reduction method 1 First step: The first step is to prepare afirst liquid with an electrode catalyst precursor (I) being dispersed inan ultrapure water. The first liquid is prepared by adding suchelectrode catalyst precursor (I) to the ultrapure water. Here, theelectrode catalyst precursor (I) is produced using a material containingchlorine (Cl) species, and exhibits a chlorine (Cl) speciesconcentration higher than a predetermined chlorine (Cl) speciesconcentration when measured by the X-ray fluorescence (XRF) spectroscopy(e.g. an electrode catalyst precursor exhibiting a chlorine (Cl) speciesconcentration value higher than 8,500 ppm or 7,600 ppm, provided that8,500 ppm or 7,600 ppm is the predetermined chlorine (Cl) speciesconcentration).

Second step: The second step is to prepare a second liquid with anelectrode catalyst precursor (II) being dispersed in the ultrapurewater. Specifically, the electrode catalyst precursor (I) contained inthe first liquid is filtrated and washed using the ultrapure water,followed by repeatedly washing the same until a filtrate obtained afterwashing has exhibited an electric conductivity ρ that is not higher thana predetermined value when measured by a JIS-standard testing method(JIS K0552) (e.g. not higher than a value predetermined within a rangeof 10 to 100 μS/cm). In this way, there is obtained the electrodecatalyst precursor (II) as well as the second liquid with such electrodecatalyst precursor (II) being dispersed in the ultrapure water.

Chlorine reduction method 2 First step: The first step is to retain aliquid containing an ultrapure water, a reductant and an electrodecatalyst precursor under at least one temperature predetermined within arange of 20 to 90° C. for a predetermined retention time. Here, theelectrode catalyst precursor is produced using a material containingchlorine (Cl) species, and exhibits a chlorine (Cl) speciesconcentration higher than a predetermined chlorine (Cl) speciesconcentration when measured by the X-ray fluorescence (XRF) spectroscopy(e.g. an electrode catalyst precursor exhibiting a chlorine (Cl) speciesconcentration value higher than 8,500 ppm or 6,000 ppm, provided that8,500 ppm or 6,000 ppm is the predetermined chlorine concentration).

Second step: The second step is to add the ultrapure water to the liquidobtained in the first step so as to prepare a first liquid where anelectrode catalyst precursor (I) contained in the liquid obtained in thefirst step is dispersed in the ultrapure water.

Third step: The third step is to filtrate and wash the electrodecatalyst precursor contained in the first liquid using the ultrapurewater, followed by repeatedly washing the same until a filtrate obtainedafter washing has exhibited an electric conductivity p that is nothigher than a predetermined first value when measured by a JIS-standardtesting method (JIS K0552). In this way, there is now obtained a secondliquid where dispersed in the ultrapure water is the electrode catalystprecursor contained in the liquid having an electric conductivity p thatis not higher than the predetermined first value.

Fourth step: The fourth step is to dry the second liquid.

Chlorine reduction method 3 First step: The first step is to retain aliquid containing an ultrapure water, a gas having hydrogen and anelectrode catalyst precursor under at least one temperaturepredetermined within a range of 20 to 40° C. for a predeterminedretention time. Here, the electrode catalyst precursor is produced usinga material containing chlorine (Cl) species, and exhibits a chlorine(Cl) species concentration higher than a predetermined chlorine (Cl)species concentration when measured by the X-ray fluorescence (XRF)spectroscopy.

The “ultrapure water” used in the chlorine reduction methods 1 to 3 is atype of water exhibiting a specific resistance R of not lower than 3.0MΩ·cm, such specific resistance R being represented by the followinggeneral formula (1) (i.e. an inverse number of the electric conductivitymeasured by the JIS-standard testing method (JIS K0552)). Further, it ispreferred that the “ultrapure water” have a water quality equivalent toor clearer than “A3” as defined in JISK 0557 “Water used for industrialwater and wastewater analysis.”

[Formula 1]

R=1/ρ  (1)

In the above general formula (1), R represents the specific resistance,and p represents the electric conductivity measured by the JIS-standardtesting method (JIS K0552).

There are no particular restrictions on the ultrapure water, as long asthe water has an electric conductivity that satisfies the relationshiprepresented by the general formula (1).

Examples of such ultrapure water include an ultrapure water producedusing an ultrapure water system from “Milli-Q series” (by Merck Ltd.);and an ultrapure water produced using an ultrapure water system from“Elix UV series” (by Nihon Millipore K.K.).

The chlorine (Cl) species contained in the electrode catalyst precursorcan be reduced by performing any one of the chlorine reduction methods 1to 3. Thus can be obtained an electrode catalyst 1 in which a bromine(Br) species concentration is adjusted to be not greater than 400 ppm,and a chlorine (Cl) species concentration is adjusted to be within therange of 0 to 900 ppm when measured by the X-ray fluorescence (XRF)spectroscopy.

X-ray fluorescence (XRF) spectroscopy The X-ray fluorescence (XRF)spectroscopy is, for example, performed in the following manner.

(1) Measurement Device

-   -   Full-automatic wavelength dispersive fluorescent X-ray analyzer        Axios (by Spectris Co., Ltd.)

(2) Measurement Condition

-   -   Analysis software: “UniQuant 5” (Semi-quantitative analysis        software employing FP (four peak method))    -   XRF measurement chamber atmosphere: Helium (normal pressure)

(3) Measurement Procedure

-   -   (i) Placing a sample-containing sample container into an XRF        sample chamber    -   (ii) Replacing an atmosphere in the XRF sample chamber with        helium gas    -   (iii) Setting the measurement condition to “UQ5 application” as        a condition required to use the analysis software “UniQuant 5”        and configuring a mode where calculation is performed in a mode        with the main component of the sample being “carbon (constituent        element of support)” and with a sample analysis result-display        format being “element,” under a helium gas atmosphere (normal        pressure)

FIG. 5 is a schematic view showing preferable embodiments of acomposition for forming gas diffusion electrode containing the electrodecatalyst of this invention; a gas diffusion electrode produced usingsuch composition for forming gas diffusion electrode; amembrane-electrode assembly (MEA) having such gas diffusion electrode;and a fuel cell stack having such membrane-electrode assembly (MEA).

As for a fuel cell stack S shown in FIG. 5, each membrane-electrodeassembly (MEA) 400 serves as a one-unit cell, and the fuel cell stack Sis configured by stacking multiple layers of such one-unit cells.

Particularly, the fuel cell stack S has a membrane-electrode assembly(MEA) 400 that is equipped with an anode 200 a, a cathode 200 b and anelectrolyte membrane 300 provided between these electrodes.

More particularly, the fuel cell stack S has a structure where themembrane-electrode assembly (MEA) 400 is sandwiched between a separator100 a and a separator 100 b.

Described hereunder are the composition for forming gas diffusionelectrode, a gas diffusion electrode 200 a, a gas diffusion electrode200 b and the membrane-electrode assembly (MEA) 400, all of which serveas members of the fuel cell stack S containing the electrode catalyst ofthis invention.

The electrode catalyst 1 can be used as a so-called catalyst inkcomponent and serve as the composition for forming gas diffusionelectrode in this invention. One feature of the composition for forminggas diffusion electrode in this invention is that this compositioncontains the aforementioned electrode catalyst. The main components ofthe composition for forming gas diffusion electrode are theabovementioned electrode catalyst and an ionomer solution. The ionomersolution contains water, alcohol and a polyelectrolyte exhibiting ahydrogen ion conductivity.

A mixing ratio between water and alcohol in the ionomer solution can beany ratio, as long as it is the kind of ratio capable of endowing aviscosity suitable for applying the composition for forming gasdiffusion electrode to the electrode. In general, it is preferred thatan alcohol be contained in an amount of 0.1 to 50.0 parts by weight withrespect to 100 parts by weight of water. Further, it is preferred thatthe alcohol contained in the ionomer solution be a monohydric alcohol ora polyhydric alcohol. Examples of a monohydric alcohol include methanol,ethanol, propanol and butanol. Examples of a polyhydric alcohol includedihydric alcohols or trihydric alcohols. As a dihydric alcohol, therecan be listed, for example, ethylene glycol, diethylene glycol,tetraethylene glycol, propylene glycol, 1,3-butanediol and1,4-butanediol. As a trihydric alcohol, there may be used glycerin, forexample. Further, the alcohol contained in the ionomer solution may beeither one kind of alcohol or a combination of two or more kinds ofalcohols. Here, the ionomer solution may also be appropriately allowedto contain an additive(s) such as a surfactant, if necessary.

For the purpose of dispersing the electrode catalyst, the ionomersolution contains a hydrogen ion-conductive polyelectrolyte as a bindercomponent for improving an adhesion to a gas diffusion layer as a partcomposing the gas diffusion electrode. Although there are no particularrestrictions on the polyelectrolyte, examples of such polyelectrolyteinclude known perfluorocarbon resins having sulfonate groups and/orcarboxylic acid groups. As an easily obtainable hydrogen ion-conductivepolyelectrolyte, there can be listed, for example, Nafion (registeredtrademark of Du Pont), ACIPLEX (registered trademark of Asahi KaseiChemical Corporation) and Flemion (registered trademark of ASAHI GLASSCo., Ltd).

The composition for forming gas diffusion electrode can be produced bymixing, crushing and stirring the electrode catalyst and the ionomersolution. The composition for forming gas diffusion electrode may beprepared using crushing and mixing machines such as a ball mill and/oran ultrasonic disperser. A crushing and stirring conditions at the timeof operating a crushing and mixing machine can be appropriatelydetermined in accordance with the mode of the composition for forminggas diffusion electrode.

It is required that the composition of each of the electrode catalyst,water, alcohol(s) and hydrogen ion-conductive polyelectrolyte that arecontained in the composition for forming gas diffusion electrode be thatcapable of achieving a favorable dispersion state of the electrodecatalyst, allowing the electrode catalyst to be distributed throughoutan entire catalyst layer of the gas diffusion electrode and improvingthe power generation performance of the fuel cell.

Particularly, it is preferred that the polyelectrolyte, alcohol(s) andwater be respectively contained in an amount of 0.1 to 2.0 parts byweight, an amount of 0.01 to 2.0 parts by weight and an amount of 2.0 to20.0 parts by weight with respect to 1.0 parts by weight of theelectrode catalyst. It is more preferred that the polyelectrolyte,alcohol(s) and water be respectively contained in an amount of 0.3 to1.0 parts by weight, an amount of 0.1 to 2.0 parts by weight and anamount of 5.0 to 6.0 parts by weight with respect to 1.0 parts by weightof the electrode catalyst. It is preferred that the composition of eachcomponent be within the abovementioned ranges, because when thecomposition of each component is within these ranges, not only a coatingfilm made of the composition for forming gas diffusion electrode willnot be spread too extensively on the gas diffusion electrode at the timeof forming the film, but the coating film formed of the composition forforming gas diffusion electrode is also allowed to have an appropriateand uniform thickness.

Here, the weight of the polyelectrolyte refers to a weight when it isdry i.e. a weight without a solvent in a polyelectrolyte solution,whereas the weight of water refers to a weight including water containedin the polyelectrolyte solution.

The gas diffusion electrode (200 a, 200 b) of this invention has a gasdiffusion layer 220; and an electrode catalyst layer 240 laminated on atleast one surface of the gas diffusion layer 220. The aforementionedelectrode catalyst is contained in the electrode catalyst layer 240equipped in the gas diffusion electrode (200 a, 200 b). The gasdiffusion electrode 200 of this invention can be used as an anode and acathode.

In FIG. 5, the gas diffusion electrode 200 on the upper side is referredto as the anode 200 a, whereas the gas diffusion electrode 200 on thelower side is referred to as the cathode 200 b for the sake ofconvenience.

In the case of the anode 200 a, the electrode catalyst layer 240 servesas a layer where a chemical reaction of dissociating a hydrogen gas sentfrom the gas diffusion layer 220 into hydrogen ions takes place due tothe function of the electrode catalyst 1 contained in the electrodecatalyst layer 240. Further, in the case of the cathode 200 b, theelectrode catalyst layer 240 serves as a layer where a chemical reactionof bonding air (oxygen gas) sent from the gas diffusion layer 220 andthe hydrogen ions that have traveled from the anode through theelectrolyte membrane takes place due to the function of the electrodecatalyst 1 contained in the electrode catalyst layer 240.

The electrode catalyst layer 240 is formed using the abovementionedcomposition for forming gas diffusion electrode. It is preferred thatthe electrode catalyst layer 240 have a large surface area such that thereaction between the electrode catalyst 1 and the hydrogen gas or air(oxygen gas) sent from the diffusion layer 220 is allowed take place tothe fullest extent.

Moreover, it is preferred that the electrode catalyst layer 240 beformed in a manner such that the electrode catalyst layer 240 has auniform thickness as a whole. Although the thickness of the electrodecatalyst layer 240 can be appropriately adjusted and there are norestrictions on such thickness, it is preferred that the electrodecatalyst layer 240 have a thickness of 2 to 200 nm.

The gas diffusion layer 220 equipped to the gas diffusion electrode 200serves as a layer provided to diffuse to each of the correspondingelectrode catalyst layers 240 the hydrogen gas introduced from outsidethe fuel cell stack S into gas flow passages that are formed between theseparator 100 a and the gas diffusion layer 220 a; and the air (oxygengas) introduced from outside the fuel cell stack S into gas passagesthat are formed between the separator 100 b and the gas diffusion layer220 b. In addition, the gas diffusion layer 220 plays a role ofsupporting the electrode catalyst layer 240 to the gas diffusionelectrode 200 so as to immobilize the electrode catalyst layer 240 tothe surface of the gas diffusion electrode 220. The gas diffusion layer220 also plays a role of improving the contact between the electrodecatalyst 1 contained in the electrode catalyst layer 240 and thehydrogen gas as well as air (oxygen gas).

The gas diffusion layer 220 has a function of favorably passing thehydrogen gas or air (oxygen gas) supplied from the gas diffusion layer220 and then allowing such hydrogen gas or air to arrive at theelectrode catalyst layer 240. For this reason, it is preferred that thegas diffusion layer 220 have a water-repellent property such that a porestructure as a microstructure in the gas diffusion layer 220 will not beblocked by the electrode catalyst 1 and a water generated at the cathode200 b. Therefore, the gas diffusion layer 220 has a water repellentcomponent such as polyethylene terephthalate (PTFE).

There are no particular restrictions on a material(s) that can be usedin the gas diffusion layer 220. That is, there can be employed amaterial(s) known to be used in a gas diffusion layer of a fuel cellelectrode. For example, there may be used a carbon paper; or a materialmade of a carbon paper as a main raw material and an auxiliary rawmaterial applied to the carbon paper as the main raw material, suchauxiliary raw material being composed of a carbon powder as an optionalingredient, an ion-exchange water also as an optional ingredient and apolyethylene terephthalate dispersion as a binder. The thickness of thegas diffusion layer can be appropriately determined based on, forexample, the size of a cell used in a fuel cell. While there are noparticular restrictions on the thickness of the gas diffusion layer, athin gas diffusion layer is preferred for the purpose of ensuring ashort diffusion distance of a reactant gas. Meanwhile, since it isrequired that the gas diffusion layer also exhibit a mechanical strengthat the time of performing coating and during an assembly process, thereis usually used a gas diffusion layer having a thickness of about 50 to300 μm, for example.

As for the gas diffusion electrodes 200 a and 200 b, an intermediatelayer (not shown) may be provided between the gas diffusion layer 220and the electrode catalyst layer 240. In such case, each of the gasdiffusion electrodes 200 a and 200 b has a three-layered structurecomposed of the gas diffusion layer, the intermediate layer and thecatalyst layer.

A production method of the gas diffusion electrode is describedhereunder.

The production method of the gas diffusion electrode includes a step ofapplying the composition for forming gas diffusion electrode to the gasdiffusion layer 220; and a step of forming the electrode catalyst layer240 by drying such gas diffusion layer 220 to which the composition forforming gas diffusion electrode has been applied. The composition forforming gas diffusion electrode contains the electrode catalyst 1 withthe catalyst components supported on the support and the ionomersolution containing a hydrogen ion-conductive polyelectrolyte, water andan alcohol(s).

The important point when applying to the gas diffusion layer 220 thecomposition for forming gas diffusion electrode is that the compositionfor forming gas diffusion electrode is to be homogeneously applied tothe gas diffusion layer 220. As a result of homogeneously applying thecomposition for forming gas diffusion electrode, there can be formed onthe gas diffusion layer 220 a coating film that has a uniform thicknessand is made of the composition for forming gas diffusion electrode.Although an application quantity of the composition for forming gasdiffusion electrode can be appropriately determined based on a mode ofusage of the fuel cell, it is preferred that the quantity be 0.1 to 0.5(mg/cm²) in terms of the amount of an active metal such as platinumcontained in the electrode catalyst layer 240, from the perspective of acell performance of a fuel cell having a gas diffusion electrode.

Next, after applying to the gas diffusion layer 220 the composition forforming gas diffusion electrode, the coating film of the composition forforming gas diffusion electrode that has been applied to the gasdiffusion layer 220 is dried so as to form the electrode catalyst layer240 on the gas diffusion layer 220. By heating the gas diffusion layer220 on which the coating film of the composition for forming gasdiffusion electrode has been formed, the water and alcohol(s) in theionomer solution contained in the composition for forming gas diffusionelectrode will be evaporated and thus disappear from the composition forforming gas diffusion electrode. As a result, in the step of applyingthe composition for forming gas diffusion electrode, the coating film ofthe composition for forming gas diffusion electrode that is formed onthe gas diffusion layer 220 becomes the electrode catalyst layer 240containing the electrode catalyst and polyelectrolyte.

The membrane-electrode assembly 400 of this invention (MembraneElectrode Assembly, abbreviated as MEA hereunder) has the anode 200 aand cathode 200 b which serve as the gas diffusion electrodes 200 usingthe electrode catalyst 1; and the electrolyte 300 dividing theseelectrodes. The membrane-electrode assembly (MEA) 400 can be produced bystacking the anode 200 a, the electrolyte 300 and the cathode 200 b inan order of anode 200 a, electrolyte 300 and cathode 200 b, and thenpressure-bonding the same.

As for the fuel cell stack S of this invention, the one-unit cell(single cell) is established with the separator 100 a (anode side) beingattached to an outer side of the anode 200 a of the membrane-electrodeassembly (MEA) 400 obtained, and with the separator 100 b (cathode side)being attached to an outer side of the cathode 200 b of themembrane-electrode assembly (MEA) 400, respectively. Further, the fuelcell stack S is obtained by integrating such one-unit cells (singlecells). Furthermore, a fuel cell system is completed by attaching aperipheral device(s) to the fuel cell stack S and assembling the same.

This invention is described in greater detail hereunder with referenceto working examples. However, this invention is not limited to thefollowing working examples.

Here, the inventors of this invention confirmed that iodine (I) specieswas not detected from the catalysts of the working and comparativeexamples, when employing the X-ray fluorescence (XRF) spectroscopy.

Further, unless otherwise noted in the description of each step of thefollowing production method, these steps were carried out under a roomtemperature and in the air.

Working Example 1

The electrode catalyst of this invention was produced through thefollowing process. The raw materials of the electrode catalyst that wereused in the working examples are as follows.

-   -   Carbon black powder: product name “Ketjen Black EC300” (by        Ketjen Black International Co.) Sodium tetrachloropalladate (II)    -   Palladium nitrate    -   Potassium chloroplatinate

As a support of the electrode catalyst, there was used a carbon blackpowder which was dispersed in a water to prepare a dispersion liquid of5.0 g/L. An aqueous solution of sodium tetrachloropalladate (II)(concentration 20% by mass) of 5 mL was then delivered by drops into andmixed with such dispersion liquid. An aqueous solution of sodium formate(100 g/L) of 100 mL was further delivered by drops into a dispersionliquid thus obtained, followed by taking the insoluble componentsthrough filtering and then washing the taken insoluble components with apure water. After performing drying, there was then obtained a palladium(core)-supported carbon with palladium being supported on carbon black.

An aqueous solution of copper sulfate of 50 mM was poured into athree-electrode electrolytic cell. A reasonable amount of thepalladium-supported carbon prepared above was then added to suchthree-electrode electrolytic cell, followed by stirring the same andthen allowing the three-electrode electrolytic cell to stand still. 450mV (pair reversible hydrogen electrode) was applied to the workingelectrode in a resting state such that copper (Cu) could uniformly coatthe palladium of the palladium-supported carbon. This is defined as acopper-palladium supported carbon.

An aqueous solution of potassium chloroplatinic acid was delivered bydrops into the solution containing the copper-palladium supported carbonwith palladium being coated by copper, the aqueous solution of potassiumchloroplatinic acid containing platinum (Pt) in an amount two-foldequivalent of the coating copper in terms of substance amount ratio. Inthis way, the copper (Cu) of the copper-palladium supported carbon wasreplaced with platinum (Pt).

After filtering a powder of the particles of such platinumpalladium-supported carbon obtained by replacing the copper (Cu) of thecopper-palladium supported carbon with platinum (Pt), without performingdrying, an ultrapure water was used to wash the same in a wet state dueto a filtrate, followed by drying the same at a temperature of 70° C.Thus, there was obtained an electrode catalyst precursor 1 {platinum(Pt)-palladium (Pd) supported carbon (core part: palladium, shell part:platinum)}, to be employed as a raw material of the electrode catalystof this invention.

The electrode catalyst precursor 1 was soaked in an aqueous solution ofsodium formate (0.0028M), and retained at a room temperature for apredetermined period of time.

Then, the electrode catalyst precursor 1 in the aqueous solution ofsodium formate was filtered and washed with ultrapure water. Thefiltering and washing with ultrapure water were repeated by apredetermined number of times. Next, the electrode catalyst washed withthe ultrapure water, was dried at a temperature of 70° C. In this way,the electrode catalyst of the working example 1, having the loadingamounts of platinum (Pt) and palladium (Pd) and the concentrations ofchlorine (Cl) species and bromine (Br) species shown in Table 1, wasobtained.

The loading amounts (% by weight) of platinum (Pt) and palladium (Pd) ofthe electrode catalyst of the working example 1 thus obtained, weremeasured by the following method. The electrode catalyst of the workingexample 1 was soaked in an aqua regia to dissolve the metal. Then,carbon as an insoluble component was removed from the aqua regia. Next,the aqua regina from which the carbon had been removed was analyzed byICP analysis.

The results of ICP analysis showed that the loading amounts of platinumand palladium were respectively 23.8% by mass and 21.9% by mass.

Working examples 2 to 3 In a similar manner to the working example 1except that the time period of soaking the electrode catalyst precursor1 in the aqueous solution of sodium formate (0.0028M) and the number oftimes of filtering and washing with ultrapure water were changed, therewere prepared electrode catalysts of the working examples 2 to 3, havingthe loading amounts of platinum (Pt) and palladium (Pd) and theconcentrations of chlorine (Cl) species and bromine (Br) species shownin Table 1.

The obtained electrode catalysts of the working examples 2 to 3 wereanalyzed by ICP analysis in a similar manner to the working example 1 tothereby measure the loading amounts of platinum and palladium.

Working examples 4 to 5 Except that the concentration of the aqueoussolution of sodium formate in which the electrode catalyst precursor 1was soaked; the time period for soaking the same in the aqueoussolution; and the number of times of filtering and washing withultrapure water were changed, there were prepared, in a similar mannerto the working example 1, electrode catalysts of the working examples 4to 5, having the loading amounts of platinum (Pt) and palladium (Pd) andthe concentrations of chlorine (Cl) species and bromine (Br) speciesshown in Table 1. It is to be noted that the concentration of theaqueous solution of sodium formate used in the working example 4 is0.0025M and the one used in the working example 5 is 0.0040M.

Working example 6 An electrode catalyst precursor 2 having differentloading amounts of platinum (Pt) and palladium (Pd) was employed insteadof the electrode catalyst precursor 1 used in the working example 1.This electrode catalyst precursor 2 was produced in a similar manner asthe electrode catalyst precursor 1 until the washing and drying step ofthe working example 1, in which after the powder of the particles of theplatinum palladium-supported carbon was filtered, the filtered powder ofthe particles of the platinum palladium-supported carbon being in a wetstate with a filtrate, was washed with ultrapure water, and dried at atemperature of 70° C.

Except that when the electrode catalyst precursor 2 was processed withthe aqueous solution of sodium formate, the concentration of the aqueoussolution of sodium formate was rendered to be 0.010M, and the period oftime for soaking the same in the aqueous solution and the number oftimes of filtering and washing with ultrapure water were changed, therewas prepared, in a similar manner to the working example 1, an electrodecatalyst of the working example 6, having the loading amounts ofplatinum (Pt) and palladium (Pd) and the concentrations of chlorine (Cl)species and bromine (Br) species shown in Table 1.

Further, ICP analysis was performed in a similar manner to the workingexample 1 to thereby measure the loading amounts of platinum andpalladium.

Working example 7 An electrode catalyst precursor 3 having differentloading amounts of platinum and palladium, was employed instead of theelectrode catalyst precursor 1 used in the working example 1. Thiselectrode catalyst precursor 3 was produced in a similar manner to theproduction step of the electrode catalyst precursor 1 until the washingand drying step of the working example 1, in which after the powder ofthe particles of the platinum palladium-supported carbon was filtered,the filtered powder of the particles of the platinum palladium-supportedcarbon being in a wet state with a filtrate, was washed with ultrapurewater, and dried at a temperature of 70° C.

Then, the powder of the electrode catalyst precursor 3 was soaked in anaqueous solution of oxalic acid (0.3M) instead of the aqueous solutionof sodium formate, and retained at a temperature of 90° C. for apredetermined period of time. Then, the powder soaked in the aqueoussolution of oxalic acid was filtered and washed with ultrapure water.Next, the powder washed with ultrapure water, was dried at a temperatureof 70° C. In this way, the electrode catalyst of the working example 7,having the loading amounts of platinum (Pt) and palladium (Pd) and theconcentrations of chlorine (Cl) species and bromine (Br) species shownin Table 1, was obtained.

Further, ICP analysis was performed in a similar manner to the workingexample 1 to thereby measure the loading amounts of platinum andpalladium.

Working examples 8 to 9 An electrode catalyst precursor 4 havingdifferent loading amounts of platinum and palladium was employed insteadof the electrode catalyst precursor 1 used in the working example 1.This electrode catalyst precursor 4 was produced in a similar manner tothe production step of the electrode catalyst precursor 1 until thewashing and drying step of the working example 1, in which after thepowder of the particles of the platinum palladium-supported carbon wasfiltered, the filtered powder of the particles of the platinumpalladium-supported carbon, being in a wet state with a filtrate, waswashed with ultrapure water, and dried at a temperature of 70° C.

Except that when the electrode catalyst precursor 4 was processed withthe aqueous solution of sodium formate, the concentration of the aqueoussolution of sodium formate was rendered to be 0.010M, and the period oftime for soaking the same in the aqueous solution and the number oftimes of filtering and washing with ultrapure water were changed, therewere prepared, in a similar manner to the working example 1, electrodecatalysts of the working examples 8 to 9, having the loading amounts ofplatinum (Pt) and palladium (Pd) and the concentrations of chlorine (Cl)species and bromine (Br) species shown in Table 1.

Further, ICP analysis was performed in a similar manner to the workingexample 1 to measure the loading amounts of platinum and palladium.

Comparative Example 1

The electrode catalyst precursor 1 used in the working example 1 wasused as it was, without filtering and washing the same by an aqueoussolution of sodium formate or the like.

Comparative Example 2

An electrode catalyst precursor 5 having different loading amounts ofplatinum and palladium was employed instead of the electrode catalystprecursor 1 used in the working example 1. This electrode catalystprecursor 5 was produced in a similar manner to the production step ofthe electrode catalyst precursor 1 until the washing and drying step ofthe working example 1, in which after the powder of the particles of theplatinum palladium-supported carbon was filtered, the filtered powderbeing in a wet state with a filtrate, was washed with ultrapure water,and dried at a temperature of 70° C.

The electrode catalyst precursor 5 thus obtained was, without filteringand washing the same with an aqueous solution of sodium formate or thelike, was used as it was, for an electrode catalyst of comparativeexample 2.

In this way, the electrode catalyst of the comparative example 2, havingthe loading amounts of platinum (Pt) and palladium (Pd) and theconcentrations of chlorine (Cl) species and bromine (Br) species shownin Table 1, was obtained.

Further, ICP analysis was performed in a similar manner to the workingexample 1 to thereby measure the loading amounts of platinum andpalladium.

Comparative Example 3

An electrode catalyst precursor 6 having different loading amounts ofplatinum and palladium was employed instead of the electrode catalystprecursor 1 used in the working example 1. This electrode catalystprecursor 6 was produced in a similar manner to the production step ofthe electrode catalyst precursor 1 until the washing and drying step ofthe working example 1, in which after the powder of the particles of theplatinum palladium-supported carbon was filtered, the filtered powderbeing in a wet state with a filtrate, was washed with ultrapure water,and dried at a temperature of 70° C.

Subsequently, the obtained electrode catalyst precursor 6 was furthersoaked in an aqueous solution of sodium formate (0.01M), and retained ata temperature of 90° C. for a predetermined period of time. Then, theelectrode catalyst in the aqueous solution of sodium formate wasfiltered and washed with ultrapure water.

In this way, the electrode catalyst of the comparative example 3, havingthe loading amounts of platinum (Pt) and palladium (Pd) and theconcentrations of chlorine (Cl) species and bromine (Br) species shownin Table 1, was obtained.

Further, ICP analysis was performed in a similar manner to the workingexample 1 to thereby measure the loading amounts of platinum andpalladium.

Concentrations of bromine (Br) species and chlorine (Cl) species X-rayfluorescence (XRF) spectrometry was employed to measure theconcentrations of the bromine (Br) species and chlorine (Cl) species ofthe electrode catalysts that were obtained in the working examples 1 to9, and the comparative examples 1 to 3. The concentrations of thebromine species and chlorine species in the electrode catalysts weremeasured using the wavelength dispersive fluorescent X-ray analyzerAxios (by Spectris Co., Ltd.). Specifically, the measurement was carriedout through the following procedure.

A measurement sample of the electrode catalyst was placed in a XRFsample container equipped in the wavelength dispersive fluorescent X-rayanalyzer. The XRF sample container in which the measurement sample ofthe electrode catalyst had been placed was then put into an XRF samplechamber, followed by replacing an atmosphere in the XRF sample chamberwith a helium gas. Later, fluorescent X-ray measurement was conductedunder the helium gas atmosphere (normal pressure).

As a software, there was used “UniQuant5” which is an analytic softwarefor use in wavelength dispersive fluorescent X-ray analyzer. Ameasurement condition(s) were set to “UQ5 application” in accordancewith the analytic software “UniQuant5,” where calculation is performedin a mode with the main component of the measurement sample of theelectrode catalyst being “carbon (constituent element of electrodecatalyst support)” and with a sample analysis result-display formatbeing “element.” Measurement results were analyzed using the analyticsoftware “UniQuant5” such that the concentrations of bromine (Br)species and chlorine (Cl) species were able to be calculated. Themeasurement results are shown in Table 1.

The catalytic activities of the electrode catalysts produced in theworking examples 1 to 9, and the comparative examples 1 to 3, wereevaluated by a rotating disk electrode method (RDE method). Thecatalytic activities of the electrode catalysts were measured by therotating disk electrode method (RDE method) in the following manner.

A powder of each of the electrode catalysts produced in the workingexamples 1 to 9 and the comparative examples 1 to 3 was taken by anamount of about 8.0 mg through measurement, and was put into a samplebottle together with an ultrapure water of 2.5 mL, followed by mixingthe same while being placed under the influence of an ultrasonicirradiation, thus producing a slurry (suspension) of the electrodecatalyst. Next, there was prepared a Nafion-ultrapure water solution bymixing an ultrapure water of 10.0 mL and a 10% by weight Nafion(registered trademark) dispersion aqueous solution (product name“DE1020CS” by Wako Chemical Ltd.) of 20 μL in a different container. TheNafion-ultrapure water solution of 2.5 mL was slowly poured into thesample bottle containing the slurry (suspension) of the electrodecatalyst, followed by thoroughly stirring the same at a room temperaturefor 15 min while under the influence of an ultrasonic irradiation, thusobtaining a composition for forming gas diffusion electrode.

FIG. 6 is a schematic diagram showing a schematic configuration of arotating disk electrode measuring device D used in the rotating diskelectrode method (RDE method).

As shown in FIG. 6, the rotating disk electrode measuring device Dmainly includes a measuring device cell 10, a reference electrode (RE)20, a counter electrode (CE) 30, a rotating disk electrode 40 and anelectrolyte solution 60.

An electrode catalyst layer X was formed on the surface of the rotatingdisk electrode 40 equipped to the rotating disk electrode measuringdevice D. Further, the catalyst of the electrode catalyst layer X wasevaluated by the rotating disk electrode method.

Particularly, there was used a rotating disk electrode measuring deviceD (model HSV110 by Hokuto Denko Corp.) employing HClO₄ of 0.1M as theelectrolyte 60, an Ag/AgCl saturated electrode as the referenceelectrode (RE) 20 and a Pt mesh with Pt black as the counter electrode(CE) 30.

A method for forming the electrode catalyst layer X on the surface ofthe rotating disk electrode 40 is described hereunder.

The composition for forming gas diffusion electrode that had beenproduced above was taken by an amount of 10 μL, and was delivered bydrops onto the surface of the clean rotating disk electrode (made ofglassy carbon, diameter 5.0 mmφ, area 19.6 mm²). Later, the compositionfor forming gas diffusion electrode was spread on the entire surface ofthe rotating disk electrode to form a uniform and given thickness,thereby forming on the surface of the rotating disk electrode a coatingfilm made of the composition for forming gas diffusion electrode. Thecoating film made of the composition for forming gas diffusion electrodewas dried under a temperature of 23° C. and a humidity of 50% RH for 2.5hours, thus forming the electrode catalyst layer X on the surface of therotating disk electrode 40.

Measurements by the rotating disk electrode method include performingcleaning inside the rotating disk electrode measuring device; anevaluation of electrochemical surface area (ECSA) prior to themeasurement; an evaluation of electrochemical surface (ECSA) before andafter an oxygen reduction (ORR) current measurement.

In the rotating disk electrode measuring device D, after soaking therotating disk electrode 40 in HClO₄ electrolyte solution 60, theelectrolyte solution 60 was purged with an argon gas for not shorterthan 30 min. Then, potential scan was performed for 20 cycles under thecondition where the scanning potential was set to be 85˜1,085 mV vsRHE,and the scanning speed was set to be 50 mv/sec.

Then, potential scan was performed for three cycles under the conditionwhere the scanning potential was set to be 50˜1,085 mV vsRHE, and thescanning speed was set to be 20 mV/sec.

After purging the electrolyte solution 60 with an oxygen gas for notshorter than 15 minutes, a cyclic voltammogram (CV) measurement wasperformed for 10 cycles under the condition where the scanning potentialwas set to be 135 to 1,085 mV vsRHE, the scanning speed was set to be 10mV/sec, and the rotation speed of the rotating disk electrode 40 was setto be 1,600 rpm. An electrical current value at a potential of 900 mVvsRHE was recorded. In addition, the rotation speed of the rotating diskelectrode 40 was individually set to be 400 rpm, 625 rpm, 900 rpm, 1,225rpm, 2,025 rpm, 2,500 rpm and 3,025 rpm, and an oxygen reduction (ORR)current measurement was carried out per each cycle. A currentmeasurement value was defined as an oxygen reduction current value (i).

Finally, the cyclic voltammogram (CV) measurement was performed forthree cycles under the condition where the scanning potential was set tobe 50 to 1,085 mV vsRHE, and the scanning speed was set to be 20 mV/sec.

The catalytic activity of the electrode catalyst was calculated using acorrection formula of mass transfer which is based on a Nemstdiffusion-layer model as shown by the following general formula (2),with the aid of the oxygen reduction current value (i) obtained aboveand a limiting current value (iL) measured in the cyclic voltammogram(CV) measurement. The calculation results of the working examples 1 to 9and the comparative examples 1 to 3 are shown in Table 1.

$\begin{matrix}{{{Formula}\mspace{14mu} 2}\mspace{641mu}} & \; \\{{ik} = \frac{{iL} \times i}{{iL} - i}} & (2)\end{matrix}$

(In the general formula (2), i represents the oxygen reduction current(ORR current) measurement value, iL represents the limiting diffusioncurrent measurement value, ik represents the catalytic activity.)

TABLE 1 Bromine Chlorine Pd/% species species Working Pt/% byconcentration/ concentration/ example by mass mass ppm ppm ik/mA Working23.8 21.9 200 900 2.51 example 1 Working 23.8 21.9 200 800 2.64 example2 Working 23.8 21.9 200 600 2.85 example 3 Working 23.8 21.9 200 5002.61 example 4 Working 23.8 21.9 200 100 2.83 example 5 Working 19.624.4 100 0 2.16 example 6 Working 23.5 21.5 100 900 2.20 example 7Working 23.7 22.0 200 600 2.30 example 8 Working 23.7 22.0 200 500 2.30example 9 Comparative 23.8 21.9 200 8400 1.90 example 1 Comparative 23.521.5 300 6100 1.68 example 2 Comparative 21.0 22.9 500 0 1.74 example 3

From Table 1, it became clear that, compared to the electrode catalystsobtained in the comparative examples 1 to 3, the electrode catalysts ofthe working examples 1 to 9 according to this invention were able toexhibit a significantly favorable catalytic activity.

The electrode catalyst of this invention is capable of demonstrating asufficient catalytic performance due to the contents of chlorine (Cl)species and bromine (Br) species being reduced to the predeterminedlevels or lower. Accordingly, this invention is a type of electrodecatalyst that can be used not only in fuel cells, fuel-cell vehicles andelectrical equipment industries such as those related to cellularmobiles, but also in Ene farms, cogeneration systems or the like. Thus,the electrode catalyst of this invention shall make contributions to theenergy industries and developments related to environmentaltechnologies.

1. An electrode catalyst having a core-shell structure comprising: asupport; a core part formed on the support; and a shell part formed tocover at least a part of a surface of the core part, wherein the shellpart includes a single-layered structure formed to cover at least a partof the surface of the core part, or a two-layered structure including afirst shell part formed to cover at least a part of the surface of thecore part, and a second shell part formed to cover at least a part ofthe surface of the first shell part, wherein in a case of thesingle-layered shell part, the shell part comprises platinum (Pt), andthe core part comprises palladium (Pd), whilst in a case of thetwo-layered shell part, the first shell part comprises palladium (Pd),and the second shell part comprises platinum (Pt), wherein aconcentration of bromine (Br) species measured by X-ray fluorescence(XRF) spectroscopy, is not greater than 400 ppm, and wherein aconcentration of chlorine (Cl) species measured by X-ray fluorescence(XRF) spectroscopy, is not greater than 900 ppm.
 2. The electrodecatalyst according to claim 1, wherein the concentration of bromine (Br)species is not greater than 300 ppm.
 3. The electrode catalyst accordingto claim 2, wherein the concentration of bromine (Br) species is notgreater than 200 ppm.
 4. The electrode catalyst according to claim 1,wherein the concentration of chlorine (Cl) species is less than 900 ppm.5. The electrode catalyst according to claim 1, wherein theconcentration of chlorine (Cl) species is equal to or greater than 0ppm.
 6. The electrode catalyst according to claim 1, wherein theconcentration of chlorine (Cl) species is not less than 100 ppm. 7-14.(canceled)
 15. The electrode catalyst according to claim 1, wherein, ina case of the two-layered shell part, the core part contains one or moremetal elements other than noble metals as a main component(s). 16.(canceled)
 17. A composition for forming a gas diffusion electrode,comprising the electrode catalyst according to claim
 1. 18. A gasdiffusion electrode comprising the electrode catalyst according toclaim
 1. 19. A membrane-electrode assembly (MEA) comprising the gasdiffusion electrode according to claim
 18. 20. A fuel cell stackcomprising the membrane-electrode assembly (MEA) according to claim 19.21. The electrode catalyst according to claim 2, wherein theconcentration of chlorine (Cl) species is less than 900 ppm.
 22. Theelectrode catalyst according to claim 3, wherein the concentration ofchlorine (Cl) species is less than 900 ppm.
 23. The electrode catalystaccording to claim 2, wherein the concentration of chlorine (Cl) speciesis equal to or greater than 0 ppm.
 24. The electrode catalyst accordingto claim 3, wherein the concentration of chlorine (Cl) species is equalto or greater than 0 ppm.
 25. The electrode catalyst according to claim4, wherein the concentration of chlorine (Cl) species is equal to orgreater than 0 ppm.
 26. The electrode catalyst according to claim 21,wherein the concentration of chlorine (Cl) species is equal to orgreater than 0 ppm.
 27. The electrode catalyst according to claim 22,wherein the concentration of chlorine (Cl) species is equal to orgreater than 0 ppm.
 28. The electrode catalyst according to claim 2,wherein the concentration of chlorine (Cl) species not less than 100ppm.
 29. The electrode catalyst according to claim 3, wherein theconcentration of chlorine (Cl) species not less than 100 ppm.
 30. Theelectrode catalyst according to claim 4, wherein the concentration ofchlorine (Cl) species not less than 100 ppm.
 31. The electrode catalystaccording to claim 21, wherein the concentration of chlorine (Cl)species not less than 100 ppm.
 32. The electrode catalyst according toclaim 22, wherein the concentration of chlorine (Cl) species not lessthan 100 ppm.